1. Field of the Invention
This invention relates to radio receiving systems which adaptively suppress interference by controlling the null directivity of the pattern of an antenna array, and more particularly to a null indicator in such a system.
2. Description of the Prior Art
Adaptive array antenna systems are currently the subject of intense interest and investigation/development for radar and communications applications. The principal reason for the interest is their ability to automatically steer nulls onto undesired sources of interference, thereby reducing output noise and enhancing the detection of desired signals. These systems usually consist of an array of antenna elements and a realtime adaptive receiver-processor which has feedback control over the element weights.
Communications and radar antenna systems using either conventional narrow band or spread spectrum modulation formats are susceptible to degradation in SNR performance caused by undesired "noise" which intrudes via the antenna sidelobes and mainlobes. The noise may consist of deliberate electronic countermeasures (ECM), friendly RF interference (RFI), clutter scatter returns, and natural noise sources. This degradation is often further aggravated by a motion of the antenna, poor siting conditions, multipath, and a changing interference environment. Adaptive array techniques offer possible solutions to these serious interference problems via their flexible capabilities for automatic null steering and notching in the spatial domain, the frequency domain, and in polarization. Adaptive nulling is considered to be the principal benefit of adaptive techniques at the present time.
It is also possible that an unfriendly source of deliberate interference may possess the capability for monitoring the transmissions from the transceiver. It is therefore desirable not only to protect the receiver from the interference, but also prevent information from the transmitter from being intercepted.
Related prior art is listed in the aforementioned co-pending application Ser. No. 222,104.